The invention concerns a tubular capacitor with variable capacitance comprising a cylindrical tube of dielectric material, a metallic outer electrode which surrounds the cylindrical tube, and an inner electrode which can axially move in the inner bore of the cylindrical tube and which abuts the inner bore, the inner electrode comprising a metallic rod.
Tubular capacitors of this type are e.g. used in NMR spectroscopic systems as described in the company leaflet “Avance—The Pulse Of Innovation” by the company Bruker BioSpin AG, Fällanden, Switzerland, April 2004.
Nuclear magnetic resonance (NMR) spectrometry is one of the most powerful methods of instrumental analytics and is used, in particular, in biological sciences and material research. A radio frequency (RF) resonator thereby irradiates RF field pulses into a sample and the response of the sample is measured and evaluated.
Irradiation of RF field pulses of minimum length yields NMR spectra of high quality. The minimum achievable length depends on the maximum value of the RF field amplitude that can be set without generating high voltage sparkovers in the RF resonator and, in particular, also in the capacitive tuning elements of the RF resonator. For this reason, the high voltage resistance (or dielectric strength) of the capacitive tuning elements limits the quality of the resulting NMR spectra. Conventional capacitive tuning elements are tubular capacitors with variable capacitance, as described above.
It is therefore the underlying purpose of the present invention to increase the dielectric strength of tubular capacitors with variable capacitance and, in particular, to improve the resolution of NMR spectrometers using these tubular capacitors.